A Kennedy representative, who spoke on condition of anonymity because the announcement was not official, said the senator was in good spirits and that doctors wanted him to get some rest.
Doctors blamed fatigue for the seizure Kennedy suffered during the Capitol Hill luncheon Tuesday after attending the swearing-in of President Barack Obama.
Senate Majority Leader Harry Reid of Nevada said Kennedy sounded good when he spoke with him both Tuesday and Wednesday.
"He's in good shape. ... He was talking last night and he was talking a lot this morning," Reid said.
The 76-year-old Massachusetts Democrat was diagnosed last May with a particularly aggressive type of brain tumor and has been treated with surgery, radiation and chemotherapy.
Kennedy's seizure at the inaugural lunch was witnessed by fellow senators. Longtime friend Sen. Chris Dodd, D-Conn., quoted Kennedy as saying, "I'll be OK, I'll see you later" as he was put into an ambulance on Tuesday.
Kennedy had appeared in good health and spirits a few hours earlier when he stepped out of the Capitol and onto the inauguration platform where Obama took the oath of office.
At the luncheon, Obama told lawmakers his prayers were with the senator and his family.
